Page 169 of the Goodheart-Willcox automotive encyclopedia, The Goodheart-Willcox Company, Inc., South Holland, Ill., 1995 describes that as fuel is burned in an internal combustion engine, about one-third of the heat energy in the fuel is converted to power. Another third goes out the exhaust pipe unused, and the remaining third must be handled by a cooling system. This third is often underestimated and even less understood.
Most internal combustion engines employ a pressurized cooling system to dissipate the heat energy generated by the combustion process. The cooling system circulates water or liquid coolant through a water jacket which surrounds certain parts of the engine (e.g., block, cylinder, cylinder head, pistons). The heat energy is transferred from the engine parts to the coolant in the water jacket. In hot ambient air temperature environments, or when the engine is working hard, the transferred heat energy will be so great that it will cause the liquid coolant to boil (i.e., vaporize) and destroy the cooling system. To prevent this from happening, the hot coolant is circulated through a radiator well before it reaches its boiling point. The radiator dissipates enough of the heat energy to the surrounding air to maintain the coolant in the liquid state.
In cold ambient air temperature environments, especially below zero degrees Fahrenheit, or when a cold engine is started, the coolant rarely becomes hot enough to boil. Thus, the coolant does not need to flow through the radiator. Nor is it desirable to dissipate the heat energy in the coolant in such environments since internal combustion engines operate most efficiently and pollute the least when they are running relatively hot. A cold running engine will have significantly greater sliding friction between the pistons and respective cylinder walls than a hot running engine because oil viscosity decreases with temperature. A cold running engine will also have less complete combustion in the engine combustion chamber and will build up sludge more rapidly than a hot running engine. In an attempt to increase the combustion when the engine is cold, a richer fuel is provided. All of these factors lower fuel economy and increase levels of hydrocarbon exhaust emissions.
To avoid running the coolant through the radiator, coolant systems employ a thermostat. The thermostat operates as a one-way valve, blocking or allowing flow to the radiator. Most prior art coolant systems employ wax pellet type or bimetallic coil type thermostats. These thermostats are self-contained devices which open and close according to precalibrated temperature values.
Coolant systems must perform a plurality of functions, in addition to cooling the engine parts. In cold weather, the cooling system must deliver hot coolant to heat exchangers associated with the heating and defrosting system so that the heater and defroster can deliver warm air to the passenger compartment and windows. The coolant system must also deliver hot coolant to the intake manifold to heat incoming air destined for combustion, especially in cold ambient air temperature environments, or when a cold engine is started. Ideally, the coolant system should also reduce its volume and speed of flow when the engine parts are cold so as to allow the engine to reach an optimum hot operating temperature. Since one or both of the intake manifold and heater need hot coolant in cold ambient air temperatures and/or during engine start-up, it is not practical to completely shut off the coolant flow through the engine block.
Practical design constraints limit the ability of the coolant system to adapt to a wide range of operating environments. For example, the heat removing capacity is limited by the size of the radiator and the volume and speed of coolant flow. The state of the self-contained prior art wax pellet type or bimetallic coil type thermostats is typically controlled only by coolant temperature.
Numerous proposals have been set forth in the prior art to more carefully tailor the coolant system to the needs of the vehicle and to improve upon the relatively inflexible prior art thermostats.
The goal of all engine cooling systems is to maintain the internal engine temperature as close as possible to a predetermined optimum value. Since engine coolant temperature generally tracks internal engine temperature, the prior art approach to controlling internal engine temperature control is to control engine coolant temperature. Many problems arise from this approach. For example, sudden load increases on an engine may cause the internal engine temperature to significantly exceed the optimum value before the coolant temperature reflects this fact. If the thermostat is in the closed state just before the sudden load increase, the extra delay in opening will prolong the period of time in which the engine is unnecessarily overheated.
Another problem occurs during engine start-up or warm-up. During this period of time, the coolant temperature rises more rapidly than the internal engine temperature. Since the thermostat is actuated by coolant temperature, it often opens before the internal engine temperature has reached its optimum value, thereby causing coolant in the water jacket to prematurely cool the engine. Still other scenarios exist where the engine coolant temperature cannot be sufficiently regulated to cause the desired internal engine temperature.
When the internal engine temperature is not maintained at an optimum value, the engine oil will also not be at the optimum temperature. Engine oil life is largely dependent upon wear conditions. Engine oil life is significantly shortened if an engine is run either too cold or too hot. As noted above, a cold running engine will have less complete combustion in the engine combustion chamber and will build up sludge more rapidly than a hot running engine. The sludge contaminates the oil. A hot running engine will prematurely break down the oil. Thus, more frequent oil changes are needed when the internal engine temperature is not consistently maintained at its optimum value.
Prior art cooling systems also do not account for the fact that the optimum oil temperature varies with ambient air temperature. As the ambient air temperature declines, the internal engine components lose heat more rapidly to the environment and there is an increased cooling effect on the internal engine components from induction air. To counter these effects and thus maintain the internal engine components at the optimum operating temperature, the engine oil should be hotter in cold ambient air temperatures than in hot ambient air temperatures. Current prior art cooling systems cannot account for this difference because the cooling system is responsive only to coolant temperature.
Prior art cooling systems have also not taken full advantage of the heat generated during combustion of the air/fuel mixture. As discussed above, approximately one third of heat generated during the combustion of the fuel/air mixture is transferred through the exhaust system. Several prior art systems have attempted to utilize this heat for improving the efficiency of an engine. For example, U.S. Pat. No. 4,079,715 discloses a prior art method for using exhaust gases to heat the intake air. Special exhaust passageways are attached to the exhaust manifold and direct the exhaust gases through or adjacent to the intake manifold thereby permitting convection of the exhaust gas heat to the intake air.
A second prior art method for utilizing the heat in the exhaust gases is disclosed on pages 229 of the Goodheart-Willcox automotive encyclopedia, The Goodheart-Willcox Company, Inc., South Holland, Ill., 1995. This method requires the incorporation of a special duct or "crossover passage" around the exhaust manifold that traps the heat which is otherwise dissipated. This trapped heated air is then routed to the intake manifold where it preheats the intake air.
These prior art methods all require the addition of special, relatively heavy ducting which must be designed to be thermally compatible with the temperatures in the exhaust gases. Additionally, these systems have all been limited to heating the intake air. Hence, the prior art methods have not utilized the heat in the exhaust gases to assist in preheating the engine and/or the engine oil.
While many of the prior art systems address the problem of cooling an internal combustion engine, none have provided a workable, cost efficient system. Accordingly, a need therefore exists for a system which optimally controls the flow of a fluid in a cooling system and which requires minimal modifications to the current engine arrangement.